Researchers led by a Brigham and Womens Hospital/Harvard Medical School team found that wheezing -- a key physiological component of asthma -- requires the interaction of genes in several locations. The work, involving multiple independent verification, demonstrates the complexity of the genetic predisposition to asthma.

The study, "Interacting genetic loci cause airway hyperresponsiveness," appears online in Physiological Genomics, published by the American Physiological Society. "We know that there are both genetic and environmental factors that cause people to develop asthma," Dr. Kate G. Ackerman, the lead researcher, stated. "We have now shown that the genetic inheritance of wheezing, one component of asthma, is extremely complex in our model. This suggests that the discovery of these genes in the human population will be difficult and will require a multi-locus approach."

After the study on the airway hyperresponsiveness (AHR) trait, Dr. David R. Beier, the laboratory leader, said: "In the specific case of asthma, hopefully these results will direct human studies to look for genetic influences in a combinatorial fashion." Using an elaborate mouse model, the researchers found that the wheezing trait unambiguously required the interaction of loci on different chromosomes, yet analysis of the individual chromosomes showed no significant association with wheezing. (A locus is a site on a chromosome where the gene for a particular trait is located.)

The Harvard researchers set out to try and determine some of the inherited aspects of asthma, which is becoming "increasingly common" and where "each patient with asthma may have a variable constellation of the different components, and each component" is itself complex. They note that "in over 10 large human studies employing genome-wide analysis to detect asthma susceptibility loci, only two significant linkages have been detected. Of these, only one has identified an association with a specific candidate gene."

Narrowing the genetic focus; "magnitude of effect"

In order to "definitively identify loci associated with a genetically complex trait" (airway hyperresponsiveness, or AHR), the team "used selection for a disease phenotype in a serial backcross" whereby offspring showing certain laboratory-induced asthma-like reactions to stimuli were then used for breeding the next generation. "In this strategy," they explain, "genomic regions that are causally associated with the trait are retained, while unassociated regions are highly likely to be lost as breeding proceeds."

After seven generations of this selective breeding, Ackerman et al. found that several regions derived from the hyperresponsive A/J mouse strain were retained in mice with elevated AHR. Contrary to expectations, they found "no apparent association between inheritance of any single locus with elevated naïve AHR in the phenotypically derived recombinant congenic line." However, according to Beier, "we were fortunate that the analytical approach we were using allowed us to check not just for association of a single locus, but for the possibility that there might be coordination or cooperation between elements involved."

Indeed, the paper reports: "We found a highly significant association with the trait when loci on both chromosomes 2 and 6 were inherited together." Put another way: "…when a pair wise scan for interactions was done, a highly significant association was found for the retained chromosome 2 and 6 regions."

Confirmation from independent project source

While noting "the theoretical possibility that this type of interaction could confound human genetic association studies," the researchers point out that: "A result with this magnitude of effect has not been previously shown using contemporary methods of linkage and association analysis."

Building on their surprising results, the Harvard group was able to test consomic Chromosome Substitution Strains (CSSs) generated by the Whitehead Institute and Case Western Reserve University. Beier noted that the Harvard mice had been selected to demonstrate AHR, but "the beauty of the CSS analysis was that even though their mice werent made specifically for airway phenotypes, they could be studied to correlate our results independently." The authors believe their study is one of the first instances in which CSSs have been utilized as a new tool to independently validate a quantitative trait linkage (QTL) analysis study.

Next steps and implications for human asthma research

The paper concludes: "The identification of genomic regions containing loci causally associated with AHR, and the demonstration that this trait requires their interaction, has important implications for the dissection of the genetic etiology of asthma in humans." The authors also identified several other areas where action is indicated:

Besides asthma, the authors say their "study has important implications for the genetic analysis of common diseases in a human population."

Since both mouse lines had a lesser degree of AHR than the parental strain, this "suggests the maximally severe phenotype depends on inheritance of more than the two interacting loci we have identified."

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